High power densities are required in order to operate sputtering processes in an economical way. The cooling of the sputtering target, however, is extremely crucial in this regard,
These days, sputtering targets are usually either directly or indirectly cooled.
A directly cooled target is schematically depicted by way of example in FIG. 2: The power relayed to the target surface 1a for the sputtering is conveyed to the back 1c of the target as a function of the thermal conductivity of the target material 1b. A cooling liquid (usually water) flows through the cooling liquid conduit 5 (usually a water conduit) in a sputtering source base body 10 and dissipates the flow of heat in accordance with the flow conditions, in order to firmly affix the target 1 to the water conduit 5, usually screws 4 are used, which extend through the target 1 and the sputtering source base body 10. It is also necessary to provide a seal 3 in order to prevent water from flowing into the vacuum chamber. Other components of a sputtering target cooling device of this type that are known to the person skilled hi the art are not shown in FIG. 2.
An indirectly cooled target is shown by way of example in FIG. 3. In this case, the cooling liquid conduit 5 is closed, producing a so-called closed cooling plate. The target 1 is fastened (e.g. screwed or clamped) to the closed cooling plate so that the back 1c of the target 1 is brought into contact with the cooling surface and the pressing of the back 1c of the target against the cooling surface enables and promotes a thermal transfer from the target to the cooling liquid. Other components of a sputtering target cooling plate device of this kind that ate known to the person skilled in the art are .not shown in FIG. 2.
Depending on the cooling .method or extreme power densities, a high temperature of the target can occur and can cause the mechanical strength of the target material to fail.
An example is the case of a target made of aluminum, which is affixed to a cooling plate as shown in FIG. 3 with screws at the edge of the cooling plate and is pressed against the solid cooling surface. When heating occurs due to the sputtering operation in. a sputtering target cooling plate device, as shown in FIG. 3, the. target may expand until tensions and mechanical distortions occur due to the clamping by means of the screws, which have a negative impact on the thermal conduction to the cooling plate, which can result in. the destruction of the target. Taking into account these boundary conditions, the power density in a sputtering target, for example made of aluminum, must be limited to the values below 10 W/cm2 and if possible, below 5 W/cm2.
Another important method of target clamping and cooling is indirect cooling by means of a movable diaphragm, as shown in Fig, 4, The target is affixed to the sputtering source base body 10 by suitable means 9 (e.g. by clamps, screws, or bayonet fittings). In the cooling conduit 5, through which a cooling liquid (as a rule water) flows, a hydrostatic pressure prevails that presses the movable diaphragm uniformly against the back 1c of the target. A movable diaphragm of this kind can, for example, be embodied as a kind of metal foil. For this reason, cooling plate devices of this kind, that are equipped with movable diaphragms are also referred to as foil cooling plate devices or simply foil cooling plates.
Between the diaphragm and the back of the target, a reduced thermal, conduction area is to be expected. This conduction area, can be improved, significantly by inserting a ductile insert foil, for example made of indium, tin, or graphite. For example, an extremely thin self-adhesive carbon film can be glued to the back of the target or to the side of the diaphragm that should be in contact with the back of the target, as described in WO2013149692 A1, in order to optimally improve the thermal conduction.
A disadvantage of this method, however, is the fact that the hydrostatic pressure subjects the target to a bending stress. At extremely high power densities and elevated temperatures, the mechanical strength of the target is often insufficient to prevent a bending and therefore destruction of the target. In particular, it is not sufficient if bayonet fittings are used as an abutment for the clamping and fixing of the target to the sputtering source body. For example, targets made of aluminum and titanium or of aluminum and chromium, which are as a rule produced using powder metallurgy, become very soft and ductile at temperatures above 200° C. As a result, at temperatures above 200° C., such targets often buckle and are destroyed.
The object of the present invention is to create a sputtering source device that makes it possible to use a cooling plate device with a movable diaphragm in which at high temperatures, the target is not destroyed by the action of the hydrostatic pressure in the cooling conduit of the cooling plate device.